Multiple isoforms of phosphoenolpyruvate carboxylase in the Orchidaceae (subtribe Oncidiinae): implications for the evolution of crassulacean acid metabolism

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Abstract

Phosphoenolpyruvate carboxylase (PEPC) catalyses the initial fixation of atmospheric CO2 into oxaloacetate and subsequently malate. Nocturnal accumulation of malic acid within the vacuole of photosynthetic cells is a typical feature of plants that perform crassulacean acid metabolism (CAM). PEPC is a ubiquitous plant enzyme encoded by a small gene family, and each member encodes an isoform with specialized function. CAM-specific PEPC isoforms probably evolved from ancestral non-photosynthetic isoforms by gene duplication events and subsequent acquisition of transcriptional control elements that mediate increased leaf-specific or photosynthetic-tissue-specific mRNA expression. To understand the patterns of functional diversification related to the expression of CAM,ppcgene families and photosynthetic patterns were characterized in 11 closely related orchid species from the subtribe Oncidiinae with a range of photosynthetic pathways from C3 photosynthesis (Oncidium cheirophorum,Oncidium maduroi,Rossioglossum krameri, andOncidium sotoanum) to weak CAM (Oncidium panamense,Oncidium sphacelatum,Gomesa flexuosaandRossioglossum insleayi) and strong CAM (Rossioglossum ampliatum,Trichocentrum nanum, andTrichocentrum carthagenense). Phylogenetic analysis revealed the existence of two mainppclineages in flowering plants, two mainppclineages within the eudicots, and threeppclineages within the Orchidaceae. Our results indicate thatppcgene family expansion within the Orchidaceae is likely to be the result of gene duplication events followed by adaptive sequence divergence. CAM-associated PEPC isoforms in the Orchidaceae probably evolved from several independent origins.

Multiple isoforms of phospho enol pyruvate carboxylase genes were sequenced from related orchid species with distinct photosynthesic types. Phylogenetic analysis indicated that CAM-associated isoforms originated from gene duplications and adaptive sequence divergence.

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